Re: BUG #13442: ISBN doesn't always roundtrip with text

> AFAICS, that isn't exactly an improvement. You're telling the user
> "if you have an ISBN column, you're screwed, and we are going to make it
> as painful as it can possibly be to get out of that situation".
>
> I think we'd be better off trying to migrate to a situation where these
> type names all still exist but they all act like ISBN13. I'm not sure
> what the stages on that journey are.

Hmm... This is more or less what the previous version was doing, including
display bugs.

So the attached v3:
- distinguishes internally between ISBN & ISBN13 (so it is clean),
but accepts values of one for the other transparently.
- displays ISBN as ISBN if possible, otherwise they are show as
ISBN13. This fixes the reported bug, and the application would
have to adapt.
- adds regression tests, including expected errors.

The good news is that there is no extension version update, it is just a
behavioral change when ISBN13 numbers are used with the ISBN type.

--
Fabien.